Collaboration is a conceptually ambiguous aspect of open education. Given inconsistent discussion about collaboration in the open education literature, this article suggests collaboration be defined and studied as a distinct open educational practice. A theoretical stance from the discipline of computer-supported collaborative learning helps conceptualize collaboration as processes of intersubjective meaning-making. Social annotation is then presented as a genre of learning technology that can productively enable group collaboration and shared meaning-making. After introducing an open learning project utilizing social annotation for group dialogue, analysis of interview and annotation data details how social annotation enabled three group-level epistemic expressions delineating collaboration as intersubjective meaning-making and as an open educational practice. A summative discussion considers how the social life of documents encourages collaboration, why attention to epistemic expression is a productive means of articulating open learning, and how to extend the study of collaboration as an open educational practice.